This application relates generally to a method and apparatus for suppressing distortion and noise in fiber optic transmission systems, and more particularly to a method and apparatus for suppressing clipping distortion and interferometric noise.
Noise in modern communication networks has been greatly reduced by improvements in the inherent noise and linearity of distributed feedback (DFB) laser transmitters. In addition, the introduction of predistortion circuitry that reduces residual non-linearities in the laser output has further improved the performance of linear fiber optic links. The demand for greater data handling capability, however, requires the linear transmission of multiple channels of information. When multi-channel signals are launched into an optical fiber, active device distortion as well as non-linear fiber effects can degrade the quality of the signal. Often, device distortion is the result of the laser output being clipped when transmitting large negative amplitude signals that occasionally occur in multichannel signals.
For example, typical state of the art optical transmitters operate with 110 channels and a modulation depth of approximately 3.5% per channel. If each of the 110 channels were to align in phase, the transmitted signal would have a peak amplitude that was 385% of the laser bias. The probability of all channels aligning in phase is negligible, so that such severe overdrive conditions are not encountered and typically need not be designed for. However, smaller, negative amplitude signal peaks do regularly occur during the transmission of multi-channel signals. Such negative amplitude signals drive the laser output power to zero. This results in distortion due to clipping of the transmitted signal.
Conventional techniques to reduce clipping distortion utilize peak detection circuits to identify amplitude spikes in the transmit signal that may result in clipping. Typically, when such a peak is detected, the laser bias is temporarily increased until the clipping event has passed. However, the laser bias must be adjusted slowly to prevent the bias control circuit from producing signals at frequencies within the transmission band. This, in turn, requires a long RF delay between the peak detector circuit and the laser transmitter. In practice, this delay is difficult to implement because of the physical size of the coaxial delay line and the RF loss of the delay line.
Another limit on the quality of the transmitted signal is interferometric noise created by double back-scattering of the optical signal within an optical fiber. Double back-scattering typically creates noise over a wide frequency spectrum, ranging from DC to the spectral width of the transmitted optical signal. Therefore, it is desirable to maximize the width of the optical spectrum so as to minimize the interferometric noise. Conventionally, the optical spectrum is increased by increasing the optical frequency modulation, or chirp of the laser or by introducing additional signals to increase the chirp.
A primary disadvantage of increasing the transmitted optical spectrum to reduce interferometric noise is a corresponding increase in the amount of intermodulation distortion due to fiber dispersions. Also, the deliberate introduction of additional signals to increase the optical spectral width of the transmitted signal may intensify the distortion caused by clipping. Therefore, it would be advantageous to provide a method and apparatus for producing a chirp generating signal that does not increase the distortion caused by clipping, as well as a method and apparatus for reducing clipping that does not require significant delay of the main RF signal.
There is therefore provided according to a presently preferred embodiment of the present invention, a method and apparatus for reducing clipping distortion and interferometric noise that do not require significant delay of the main RF signal. In a preferred embodiment of the present invention, an information carrying signal with high amplitude peaks drives the RF input of a laser as well as the input of a laser bias control circuit. The output of the laser bias control circuit is coupled to the bias input of the laser. The laser bias control circuit directly modulates the laser with a low frequency signal that is proportional to the frequency of occurrence and intensity of peaks in the information carrying signal that are likely to cause the laser to clip.
Alternatively, the laser bias adjustment circuit is also applied to a voltage variable RF attenuator. The voltage variable attenuator serves to substantially cancel or reduce amplitude modulations of the RF carriers due to the modulation of the laser bias current.
These and other aspects of the present invention will be more readily understood when considered in connection with the drawings and the following detailed description.